Numerical Investigation on Unsteady Characteristics in Different Rim Seal Geometries: Part A

Secondary sealing flow is of great importance in the turbine disk cooling and sealing system. The amount of cooling air extracted from the compressor is crucial to engine efficiency. To determine a minimum amount of cooling air, the flow characteristic of the rim seal should be investigated. Numerical simulation is carried out to investigate the flow field near the rim seal region. Both RANS and URANS numerical simulation methods are used in the commercial CFD code ANSYS CFX to analyze axial and radial rim seals. In the simulation, a 1/33 sector is selected as computing region to simulate the flow field and the SST turbulent model is used. The steady and unsteady simulation results of pressure distribution and seal efficiency are analyzed and compared. The computed results show that due to the different geometry configuration, the pressure distribution also shows inconsistency. Unsteady phenomena are observed in both axial and radial type of rim seals. Radial sealing lip can suppress the inherent unsteadiness and interaction between main flow and sealing flow, thus showing higher sealing efficiency. Comparing to steady results using the RANS method; unsteady simulation, using the URANS method, can capture the pressure difference and seal efficiency fluctuation at the disk rim more efficiently. Also, the interaction between the rotor and stator is considered in unsteady simulation, so the unsteady simulation is recommended. The results obtained in the current paper are useful to the investigation and design of turbine rim seals.

An Integrated Approach to Simulate Gas Turbine Secondary Air System

One of the most critical parts of a modern gas turbine that its reliability and performance has a great influence on cycle efficiency is the secondary air system (SAS). Modern systems functions to supply not only cooling air flow for turbine blades and vanes but sealing flow for bearing chambers and turbine segments as well as turbine disks’ purge flow in order to eliminate hot gas ingestion. Due to the various interactions between SAS and main gas, consideration of the former is substantially crucial in design and analysis of the whole engine. Geometrical complexities and centrifugal effects of rotating blades and disks, however, make the flow field and heat transfer of the problem so complicated AND too computationally costly to be simulated utilizing full 3-D CFD methods. Therefore, developing 1-D and 0-D tools applying network methods are of great interests. The present article describes a modular SAS analysis tool that is consisted of a network of elements and nodes. Each flow branch of a whole engine SAS network is substituted with an element and then, various branches (elements) intersect with each other just at their end nodes. These elements which might include some typical components such as labyrinth seals, orifices, stationary/rotating pipes, pre-swirls, and rim-seals, are generally articulated with characteristic curves that are extracted from high fidelity CFD modeling using commercial software such as Flowmaster or ANSYS-CFX. Having these curves, an algorithm is developed to calculate flow parameters at nodes with the aid of iterative methods. The procedure is based on three main innovative ideas. The first one is related to the network construction by defining a connectivity matrix which could be applied to any arbitrary network such as hydraulic or lubrication networks. In the second one, off-design SAS calculation will be proposed by introducing some SAS elements that their characteristic non-dimensional curves are influenced by their inlet total pressure. The last novelty is the integration of the blades coolant calculation process that incorporates external heat transfer calculation, structural conduction and coolant side modeling with SAS network simulation. Finally, SAS simulation of an industrial gas turbine is presented to illustrate capabilities of the presented tool in design point and off-design conditions.

Influence of Inflow Distributions on Swirling Flow in Rotor-Stator Cavity With Centripetal and Centrifugal Superposed Flow

The disk cavity of the rotor-stator system is one of important parts of the aero-engine air system. The rotating speed of airflow determines the relative velocity between the airflow and the rotating assembly, so the maintaining characteristic for swirling flow of the rotor-stator cavity will affect the windage of the rotating assembly in the cavity, and the exporting characteristic for swirling flow of the rotor-stator cavity will affect the windage of the rotating assembly in the downstream chamber. Dual-inlet rotor-stator disk cavity is a typical structure of aero-engine. But there are few reports about the flow structure in the dual-inlet rotor-stator disk cavity in the open literature. In this paper, the influence of inflow distributions on maintaining and exporting characteristics for swirling flow was investigated by numerical simulation, aiming at the interaction between the two inflows. The SST turbulence model was well validated against published experimental results. The simulation results show that there are two flow regions in the rotor-stator cavity from the perspective of meridian plane, and there is a rotating vortex in each flow region. In order to quantify the rotational capacity of the airflow, the angular momentum coefficient was defined. Under different inflow distribution ratios, the sizes of the dominant regions of the two vortices in the cavity get varied, which results in the variation of maintaining and exporting characteristics for swirling flow. It was also found that inflow distributions have the same effect on maintaining and exporting characteristics for swirling flow under different rotational Reynolds numbers and different throughflow coefficients, but the capacity of maintaining and exporting swirling flow are different.

Development of New Single and High-Density Heat-Flux Gauges for Unsteady Heat Transfer Measurements for a Rotating Transonic Turbine

This paper presents the development and implementation of a new generation of double-sided heat-flux gauges at The Ohio State University Gas Turbine Laboratory (GTL) along with heat transfer measurements for film-cooled airfoils in a single-stage high-pressure transonic turbine operating at design corrected conditions. Double-sided heat flux gauges are a critical part of turbine cooling studies, and the new generation improves upon the durability and stability of previous designs while also introducing high-density layouts that provide better spatial resolution. These new customizable high-density double-sided heat flux gauges allow for multiple heat transfer measurements in a small geometric area such as immediately downstream of a row of cooling holes on an airfoil. Two high-density designs are utilized: Type A consists of 9 gauges laid out within a 5 mm by 2.6 mm (0.20 inch by 0.10 inch) area on the pressure surface of an airfoil, and Type B consists of 7 gauges located at points of predicted interest on the suction surface. Both individual and high-density heat flux gauges are installed on the blades of a transonic turbine experiment for the second build of the High-Pressure Turbine Innovative Cooling program (HPTIC2). Run in a short duration facility, the single-stage high-pressure turbine operated at design-corrected conditions (matching corrected speed, flow function, and pressure ratio) with forward and aft purge flow and film-cooled blades. Gauges are placed at repeated locations across different cooling schemes in a rainbow rotor configuration. Airfoil film-cooling schemes include round, fan, and advanced shaped cooling holes in addition to uncooled airfoils. Both the pressure and suction surfaces of the airfoils are instrumented at multiple wetted distance locations and percent spans from roughly 10% to 90%. Results from these tests are presented as both time-average values and time-accurate ensemble averages in order to capture unsteady motion and heat transfer distribution created by strong secondary flows and cooling flows.

Feasibility Study of a Radical Vane-Integrated Heat Exchanger for Turbofan Engine Applications

In the present study, a compact heat exchanger for cryogenically fueled gas turbine engine applications is introduced. The proposed concept can be integrated into one or various vanes that comprise the compression system and uses the existing vane surface to reject core heat to the cryogenic fuel. The requirements for the heat exchanger are defined for a large geared-turbofan engine operating on liquid hydrogen. The resulting preliminary conceptual design is integrated into a modified interconnecting duct and connected to the last stage of a publicly available low-pressure compressor geometry. The feasibility of different designs is investigated numerically, providing a first insight on the parameters that govern the design of such a component.

Acquisition and Processing Considerations for Infrared Images of Rotating Turbine Blades

As designers aim to increase efficiency in gas turbines for aircraft propulsion and power generation, spatially-resolved experimental measurements are needed to validate computational models and compare improvement gains of new cooling designs. Infrared (IR) thermography is one such method for obtaining spatially-resolved temperature measurements. As technological advances in thermal detectors enable faster integration times, surface temperature measurements of rotating turbine blades become possible to capture including the smallest features. This paper outlines opportunities enabled by the latest IR detector technologies for capturing spatially-resolved rotating blade temperatures, while also addressing some of the challenges of implementing IR for turbine rigs such as the one in the Steady Thermal Aero Research Turbine (START) Laboratory. This paper documents critical steps in achieving accurate measurements including calibration, integration times, spatial noise, and motion blur. From these results, recommendations are provided for achieving accurate IR measurements collected in a rotating turbine facility to study film cooling.

Exploring Applicability of Acoustic Heat Transfer Enhancement Across Various Perturbation Elements

The requirements of improved heat transfer performance on turbine surfaces and internal cooling passages drive the research into exploring new methods for efficiency enhancements. Addition of ribbed structures inside the cooling ducts has proven to be most practical, which increases heat transfer from surfaces to fluid flow at the cost of some pressure loss. Many active and passive methods have been proposed for enhancing the heat transfer, where acoustic excitation has been recently shown to be an effective option. Moreover, the existing pressure fluctuations due to rotor-stator interactions can also be utilized as a source of excitation. However, the sensitivity of the phenomenon to various flow and geometric parameters has not been fully characterized. The present study investigates various aspects of convective heat transfer enhancement and turbulent flow modulation caused by acoustic forcing on separating and reattaching flow over isolated rib obstacles. A parametric study is conducted; rib obstacles of various sizes and shapes (including rectangular, squared, triangular, semi-cylindrical, etc.) are installed in a low-speed, fully turbulent wind tunnel and measurements are taken at different velocities and excitation frequencies. Static pressure and spatially resolved surface temperature measurements are performed to quantify the ramifications of acoustic excitation on the wetted wall. Within the favorable Strouhal number range of 0.1–0.25, an optimum value of 0.16 is observed. It is shown that triangular ribs are more prone to acoustic heat transfer enhancement than rectangular or cylindrical perturbations. A linear correlation between static pressure recovery rate and acoustic heat transfer enhancement is observed, which is invariant to change in size/shape of the rib as well as flow and excitation parameters.

Measurement and Analysis of Buoyancy-Induced Heat Transfer in Aero-Engine Compressor Rotors

The flow inside cavities between co-rotating compressor discs of aero-engines is driven by buoyancy, with Grashof numbers exceeding 1013. This phenomenon creates a conjugate problem: the Nusselt numbers depend on the radial temperature distribution of the discs, and the disc temperatures depend on the Nusselt numbers. Furthermore, Coriolis forces in the rotating fluid generate cyclonic and anti-cyclonic circulations inside the cavity. Such flows are three-dimensional, unsteady and unstable, and it is a challenge to compute and measure the heat transfer from the discs to the axial throughflow in the compressor. In this paper, Nusselt numbers are experimentally determined from measurements of steady-state temperatures on the surfaces of both discs in a rotating cavity of the Bath Compressor-Cavity Rig. The data are collected over a range of engine-representative parameters and are the first results from a new experimental facility specifically designed to investigate buoyancy-induced flow. The radial distributions of disc temperature were collected under carefully-controlled thermal boundary conditions appropriate for analysis using a Bayesian model combined with the equations for a circular fin. The Owen-Tang buoyancy model has been used to compare predicted radial distributions of disc temperatures and Nusselt numbers with some of the experimentally determined values, taking account of radiation between the interior surfaces of the cavity. The experiments show that the average Nusselt numbers on the disc increase as the buoyancy forces increase. At high rotational speeds the temperature rise in the core, created by compressibility effects in the air, attenuates the heat transfer and there is a critical rotational Reynolds number for which the Nusselt number is a maximum. In the cavity, there is an inner region dominated by forced convection and an outer region dominated by buoyancy-induced flow. The inner region is a mixing region, in which entrained cold throughflow encounters hot flow from the Ekman layers on the discs. Consequently, the Nusselt numbers on the downstream disc in the inner region tend to be higher than those on the upstream disc.

Process Automation for Aero Engine Secondary Air System Seal Design

Secondary air system seals are crucial in aero engine design as they have a direct impact on specific fuel consumption. Their behavior is affected by several aspects of the physics of the system: the air system, the engine thermal physics, the effect of flight loads and several other effects. As a consequence, their design is a complex and iterative process, which is highly dependent on the location of the seal in the engine, on the system requirements and on the system behavior. This paper describes a methodology for multi-disciplinary assessment of secondary air system seals within an engine environment and supports standard seal design, trade-off studies on novel concepts and system-level optimization. Defining the seal design intent for a specific engine location in the form of objectives, it is possible to embed process automation into traditionally manual multi-disciplinary design processes. This allows transforming modelling and simulation tools, which typically provide predictions for a specific seal design over reference cycles, into design and optimization tools, which can provide the optimum seal design for a specific set of requirements. This approach provides predictive models of both seal performance and performance degradation and is capable of taking into account all sources of variation, for instance manufacturing variations or engine operating conditions, delivering a robust design, specific to the engine location. The methodology enables a holistic approach to system and sub-system design and provides a deeper understanding of the impact of the seal onto system and of the system onto the seal, allowing optimization of the overall solution and informing the business case for introduction of different sealing strategies. Examples of the application of this methodology are provided for both labyrinth seals and leaf seals.

Numerical Analysis of the Phase-Change Heat Transfer Inside a Pulsating Heat Pipe With Overcritical Number of Turns

The Pulsating Heat Pipe (PHP) is a promising device in the family of heat pipes. With no need for a wick, they exhibit a high heat transfer to weight ratio. Moreover, the wickless design removes limits commonly associated with conventional heat pipes, increasing the maximum power transfer per single heat pipe. These peculiarities make it an ideal candidate for many high power applications. Nonetheless, there is though only partial knowledge on the driving mechanism, which restricts prediction accuracy. Most Pulsating Heat Pipe studies rely on experiments to test configurations, while simulations usually depend on semi-empirical correlations or adaptations of reduced theoretical models. Experiments provide detailed data for a particular geometry in lab fixed conditions, but it offers limited flexibility to test alternative configurations. Semi-empirical models use previous experimental data to create non-dimensional formulations. Though approaching an increased set of conditions, correlations apply with reasonable accuracy only to a small range, outside of which the prediction ability progressively falls. High order numerical analysis such as Computational Fluid Dynamics (CFD) modeling could potentially provide full visualization, but due to the complex flow behavior, previous studies used this method only in simple configurations with a small number of turns. The present research will expand the potential of this modeling technique by presenting the CFD analysis of a complex Pulsating Heat Pipe configuration. The importance of this study lies in the fact that this configuration, with a number of turns greater than a critical parameter, shows a reduced sensitivity to gravity and is therefore particularly important for applications where restrictions on installations make the positioning sub-optimal. The research simulates using a CFD commercial software a two-dimensional Pulsating Heat Pipe with sixteen turns. The heat pipe, with a 2 mm internal diameter, is filled with water at 50% of mass. To visualize the oscillation pattern of liquid and vapor slugs and plugs inside the Pulsating Heat Pipe, the model performs a transient analysis on the device. A Volume of Fluid (VOF) solver for multiphase analysis, coupled with the Lee model for evaporation and condensation mass transfer, calculates the interactions between the liquid and the gas phase inside the tube. The study follows the geometric and operational conditions from previous experiments. The analysis regards a Pulsating Heat Pipe operating in a vertical position with the condenser section placed in the upper sector. During the initial operations, the system flow distribution fluctuates between different flow modes as the fluid slugs and plugs structure forms. After stabilizing the heat transfer results well agree with the tested values. Moreover, the increased resolution allows us to fully visualize the internal operation, retrieving additional information on the temperature and ratio of liquid and gas phase along the heat pipe.